Electromagnetic field enhancement from a localized surface plasmon resonance is the major basis for surface enhanced Raman scattering (SERS), which has attracted much scientific and engineering interest because of potential use in highly sensitive, real time, non-destructive, and multiplexing molecular detection. One vital prerequisite for SERS-based molecular detection is to reliably generate ‘Raman hot spots’ for amplifying the inherently small cross-section of Raman scattering. Since the first demonstration of single molecule detection, the salt or evaporation induced aggregation of colloidal Au or Ag nanoparticles in liquids or the Langmuir-Blodgett assembly of nanoparticles on substrates have been widely used as convenient methods for creating Raman hot spots. These bottom-up routes, however, are usually accompanied by loss of reproducibility and yield. For example, recent analysis of the distribution of SERS enhancement from Ag thin films deposited on self-assembled nanoparticles shows the hottest sites (enhancement factor>109) account for only 0.006% of the total. Extensive studies on lithographically patterned plasmonic nanostructures have also been done in order to generate more uniform and reproducible hot spots, however, these structures also have critical issues. Typically, the patterned nanoscale gap structures, such as plasmonically coupled nano-rods, disks, or prisms, cannot maintain their well-controlled gap distance when they are released from the substrate. This limits their use for biomedical applications of SERS-based in-vitro or in-vivo imaging.
What is needed are nanoparticles, and a method of fabricating nanoparticles, designed to produce strong electromagnetic field enhancements near spatially sharp features of each nanoparticle through the use plasmonic metals and rationally fabricated nano-shapes. These synthetic nanoparticles are formed by direct-fabrication by top-down physical routes, in which materials are deposited in a nano-patterned polymer template, allowing for exquisite control over material composition, multilayer structure, particle size, and shape which are hardly achievable with chemical nanoparticle synthesis.